(1) Field of the Invention
The present invention relates to a method of making a resistor in an integrated circuit with a limited layout area, and more particularly to a method of making a highly packed static random access memory (SRAM) cell with a resistor.
(2) Description of the Prior Art
Referring first to FIG. 1, which illustrates a method of making the conventional resistor in a SRAM cell. First, in an N-type semiconductor substrate 50 is formed a P-tub 52, on which is overlaid a field oxide layer 54. Then, field effect transistors (FETs) are formed, which comprise gate oxide layers 56, gate electrodes 58, capped nitride 59, silicon nitride or silicon oxide spacers 62, N.sup.- lightly doped areas 60 and N.sup.+ heavily doped source/drain regions 64.
After a first insulating layer 66 is deposited and etched by the photolithography and etching techniques to form a bit line contact hole, a bit line 67A/67B is subsequently formed. The bit line 67A/67B is usually made of polycide, which is comprised of polysilicon 67A and tungsten suicide 67B. Then, a second insulating layer 70 is formed upon the entire structure. After the second insulating layer 70 is planarized, the second insulating layer 70 and the first insulating layer 66 are etched by the photolithography and plasma etching techniques to form a contact hole. Then, a polysilicon layer 72 is deposited across the contact hole and etched by the photolithography and etching techniques to form a polysilicon load resistor 72.
A third insulating layer 74 and a fourth insulating layer 76 are subsequently formed and etched by the photolithography and etching techniques to form a metallurgy contact hole. Then a first metal interconnection 78/80/82 is formed, which comprises titanium nitride 78, tungsten plug 80, and aluminum alloy 82. This structure is well-known.
A fifth insulating layer 84 is then deposited and etched by the photolithography and plasma etching techniques to form a via hole (not shown). Next, a second metal interconnection 86 is formed across the via hole and electrically connects with the first metal interconnection 78/80/82, which also comprises the well-known structure of titanium nitride, tungsten plug, and aluminum alloy. Then, a sixth insulating layer 88 and a seventh insulating layer 90 are deposited as passivation layers. The sixth insulating layer 88 is made of silicon dioxide formed by plasma enhanced chemical vapor deposition (PECVD) at the temperature of between 300 to 400.degree. C. with tetraethylorthosilicate (TEOS) reactant gas. The seventh insulating layer 90 is made of silicon nitride formed by PECVD at the temperature of between 300 to 400.degree. C.
However, as depicted in FIG. 1, the second insulating layer 70 and the first insulating layer 66 are very thin. When making resistor 72, it is necessary to extend the length of the resistor 72 in the horizontal direction to acquire higher resistance values, that wastes a lot of layout areas. In this way, the packing density of the SRAM cell is limited.
The present invention provides a method of making a resistor utilizing a polysilicon plug with a high aspect ratio, which extends in the vertical direction. Therefore, the layout area can be saved to accomplish higher packing density of a SRAM cell.
Within the prior art, there are some references addressing different resistor structures and methods of making the same, however, none appear to be directed to the specific application of the present invention, namely the use of the polysilicon plug formed with a high aspect ratio to make the resistor with a small layout area. For example, U.S. Pat. No. 4,835,588 to Nawata et al. provides a transistor with a plurality of resistors regions commonly connected through an emitter wiring electrodes. The resistor regions are formed in an elongated shape. U.S. Pat. No. 5,400,277 to Nowak discloses a method of making the resistor by first forming a trench extending from top of the wafer through an isolation region of the wafer to a silicon base of the wafer. A resistive layer is then formed in the trench to form the resistor.